System For Laser-Based Digital Marking Of Objects With Images Or Digital Image Projection With The Laser Beam Shaped And Amplified To Have Uniform Irradiance Distribution Over the Beam Cross-Section

ABSTRACT

A laser marking system including a laser apparatus to supply a laser beam having a non-Gaussian irradiance distribution over a beam cross-section, the non-Gaussian irradiance distribution of the laser beam has a substantially uniform irradiance distribution over the beam cross-section, a spatial light modulator coupled to receive the laser beam, the spatial light modulator is controlled to generate an output laser beam including an optical pattern across the beam cross-section to mark a target object with the data code matrix, and an optical amplifier coupled to the spatial modulator to receive the laser beam output from the spatial light modulator and generate an amplified laser beam containing the same optical pattern as generated by the spatial light modulator, the amplified laser beam from the optical amplifier having a substantially uniform amplification across the cross-section of the beam, the amplified beam maintaining the substantially uniform irradiance distribution over its beam cross-section.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional No. 61/372,197filed Aug. 10, 2011, and is incorporated herein by reference in itsentirety. This application also claims priority to and is aContinuation-In-Part of U.S. application Ser. No. 12/204,390 filed Sep.4, 2008 and is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention is generally related to laser image amplificationmarking systems, and, more particularly, to a system comprising a laserbeam-shaping arrangement to have a spatially uniform irradiance for theinitial beam, a Spatial Light Modulator (SLM) and an optical amplifierconfigured to provide an amplified beam having a spatially uniformirradiance.

BACKGROUND OF THE INVENTION

It is known that optical representation of data, such asmachine-readable bar codes, logos, etc., can be attached, placed ormarked on products by various means, such as industrial ink jetprinting, electrolytic chemical etching and laser markings. The data canserve multiple purposes including product identification, track andtrace information, anti-counterfeit detection, etc. Laser-based markingdevices do not require inks, solvents and other chemicals and thus canprovide a marking implementation that is comparatively less expensivewith lower operating costs and more environmental friendly, such aswithout generating hazardous solvent emissions. Moreover, thelaser-based markings are generally longer lasting, have betterresolution quality and do not wear off easily.

A majority of presently available laser-based marking systems usegalvanometer-based optical scanning technology where a laser beam isscanned across the object to be marked. Although the technology has madeadvances in terms of speed and performance, placing a 2-D bar matrixcode or a high resolution image on the object can be challenging. Forexample, placing a high resolution image with an example resolution of1024 by 768 pixels would require 786,432 marking operations in agalvanometer-based system. On the other hand, a laser marking systembased on a Spatial Light Modulator (SLM), such as aMicro-Electro-Mechanical-System (MEMS)-based digital micromirror device(DMD) or Liquid Crystal Devices (LCD) based SLMs etc., cansimultaneously process a complete code matrix or a high resolution imagein a single operation. The ability of these devices to project variabledata at high speeds make them an excellent choice for laser markingsystems performing serialized data bar coding specially two-dimensional(2-D) data matrix codes. However, certain drawbacks can arise duringtheir operation. In SLM based marking, the total energy/power of thelaser is shared among all the pixels as against a scanner based systemwhere all the power is focused at one pixel. Thus, in order to mark theintended the target, the overall energy/power of the laser has to behigher. However, the overall power cannot be increased indefinitely andis limited by the damage threshold of the SLM. Several approaches havebeen proposed that try to address these drawbacks, but with limitations.

One prior art laser marking system (U.S. Pat. No. 6,836,284) is believedto use a digital micromirror device (DMD), operating as a SLM thatrequires a beam expansion and beam contraction mechanism (i.e., requiresoptics adapted to intentionally affect the size of the cross-section ofthe beam) to avoid damage to the DMD. The beam expansion spreads theoptical power of an incident beam over a larger area and thus reducesthe irradiance (power per unit area) so that the DMD is not damaged.After reflection from the DMD, the beam is contracted again to increasethe irradiance. The system described in the foregoing patent, however,is somewhat impractical since the physical dimensions and thecross-sectional area of available micromirror devices are relativelysmall (in the order of few square cm). Due to their smallcross-sectional area, the present Applicant believes that the spatialprofile of the incident laser beam cannot be expanded beyond a certainmagnification limit (L), as shown in FIG. 14. That is, the irradiance ofthe incident laser beam cannot be reduced by a factor greater than L.Any further magnification (M>L) would result in part of the beam to missthe device. Thus, in practice, Applicant believes that the markingsystem proposed in the foregoing patent would not be effective forapplications requiring optical intensities L times higher than the DMDdamage threshold. Also, in applications where large sized marks arerequired, with mark size being comparable to the size of a DMD/SLMitself, Applicants submits that the beam expansion and contractionmechanism would be useless.

Another solution suggested in prior art to avoid damage to the SLM, asdisclosed in U.S. Pat. No. 7,058,109, appears to involve standard lasersand amplifiers, which produce optical beams characterized by either aGaussian irradiance distribution or other non-uniform distribution. As aresult, the irradiance of the laser beam at the target surface would beGaussian or non-uniform, which could adversely affect the quality of themarks, whether non-ablative or ablative. For example, in the case ofmarks based on ablative marking (as may involve a metal foil), therelatively high irradiance at the center of a Gaussian beam, as comparedto substantially lower levels at the outer edges of the beam, couldresult in a melting of the foil at the center of the mark leaving ahole, whereas the irradiance level at the beam edge may not besufficiently high to ablate the foil metal. For example, in the case ofnon-ablative marking (e.g., photo-chemical change of a coating underlaser irradiance), the relatively high irradiance at the center of theGaussian beam might lead to heat conduction to areas on the target thatare not designated for marking, and the relatively weak irradiance atthe outer edges of the beam may not be conducive to a color change. If a2-D data matrix code is marked on a target (ablative or non-ablative)the non-uniform irradiance distribution would ruin the contrast ratio ofthe code and a code reader would be unable to read it. Thus, theirradiance distribution across the laser beam cross-section should besufficiently uniform so that the information, i.e., the projected imageis within acceptable quality levels, e.g., readable marks without lossof information. Accordingly, it is desirable to provide a practical andreliable SLM based laser marking system that provides a cost-effectivesolution to overcome the above-described issues.

BRIEF DESCRIPTION OF THE INVENTION

Exemplary embodiment of the present invention disclose a laser markingsystem for marking of objects with images, or digital image projection,with a laser beam shaped and amplified to have uniform irradiancedistribution over the beam cross-section.

The laser marking system is disclosed comprising a laser apparatusconfigured to supply a laser beam having a non-Gaussian irradiancedistribution over a beam cross-section, wherein the non-Gaussianirradiance distribution of the laser beam comprises a substantiallyuniform irradiance distribution over the beam cross-section. A spatiallight modulator optically coupled to receive the laser beam, wherein thespatial light modulator is controlled to generate an output laser beamcomprising an optical pattern across the beam cross-section to mark atarget object with the data code matrix is also provided. Further partof the laser marking system is an optical amplifier coupled to thespatial modulator to receive the laser beam output from the spatiallight modulator and generate an amplified laser beam containing the sameoptical pattern as generated by the spatial light modulator, theamplified laser beam from the optical amplifier having a substantiallyuniform amplification across the cross-section of the beam, thereby theamplified beam maintaining the substantially uniform irradiancedistribution over its beam cross-section.

In another exemplary embodiment, the laser marking system comprises ameans for generating a laser beam having a non-Gaussian irradiancedistribution over a beam cross-section, wherein the non-Gaussianirradiance distribution of the laser beam comprises a substantiallyuniform irradiance distribution over the beam cross-section. A spatiallight modulator optically coupled to receive the laser beam, wherein thespatial light modulator is controlled to generate an output laser beamcomprising an optical pattern containing a data code matrix, a logo, orboth across the beam cross-section to mark a target object with the datacode matrix is also provided. Further provided is an optical amplifyingmeans for generating an amplified laser beam containing the same opticalpattern as generated by the spatial light modulator, the amplified laserbeam from the optical amplifying means having a substantially uniformamplification across the cross-section of the beam, thereby theamplified beam maintaining the substantially uniform irradiancedistribution over its beam cross-section.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present invention will be more readily understood and thevarious advantages and uses thereof more readily appreciated, whenconsidered in view of the following detailed description when read inconjunction with the following figures, wherein:

FIG. 1 shows a schematic representation of an example embodiment of alaser image amplification system (LIAS) using a laser source configuredto produce a beam having a flat-top profile in accordance with aspectsof the present invention;

FIG. 2 shows a schematic representation of another example embodiment ofa LIAS that in accordance with further aspects of the present inventionuses a Gaussian to flat-top converter configured to supply a beam havinga flat-top profile;

FIG. 3 shows a schematic representation of another example embodiment ofa LIAS that in accordance with further aspects of the present inventionmay be used for marking objects with a logical complement of originalinformation such as a code/image;

FIG. 4 illustrates an example Gaussian-shaped irradiance distribution ofa laser beam;

FIG. 5 illustrates an example non-Gaussian (e.g., idealized flat-top)irradiance distribution of a laser beam;

FIG. 6 illustrates an example non-ideal flat-top irradiancedistribution, as may be used in a practical laser marking systemembodying aspects of the present invention;

FIG. 7 illustrates an example of a 2-D data code matrix as may beconstructed by a spatial light modulator (SLM), as may be used by alaser marking system in accordance with aspects of the presentinvention;

FIG. 8 shows an example resulting irradiance profile of a beam encodedwith the example 2-D data code matrix shown in FIG. 7;

FIG. 9 shows a respective optical amplification profiles that may beused to comparatively conceptualize aspects of the present inventionrelative to optical amplification techniques generally used in prior artsystems;

FIG. 10 shows another respective optical amplification profiles that maybe used to comparatively conceptualize aspects of the present inventionrelative to optical amplification techniques generally used in prior artsystems;

FIG. 11 shows yet another respective optical amplification profiles thatmay be used to comparatively conceptualize aspects of the presentinvention relative to optical amplification techniques generally used inprior art systems;

FIG. 12 shows an example optical amplifier architectures as may be usedto provide an amplified laser beam having a substantially uniformamplification across the cross-section of the beam, which allowsmaintaining a substantially uniform irradiance distribution over thebeam cross-section;

FIG. 13 shows another example optical amplifier architectures as may beused to provide an amplified laser beam having a substantially uniformamplification across the cross-section of the beam, which allowsmaintaining a substantially uniform irradiance distribution over thebeam cross-section;

FIG. 14 graphically illustrates some of the practical limitations of aprior-art laser marking system based on a beam expansion-contractionmechanism that intentionally changes the size (i.e., cross-section) of alaser beam used by such a system;

FIG. 15 shows a manner in which tilting the SLM through a certain angleallows the coupling of a majority of the laser power reflected off theSLM in one of the diffraction orders produced by the SLM;

FIG. 16( a) shows how the uniform amplification can be performed beforethe imaging optics;

FIG. 16( b) shows how the uniform amplification can be performed afterthe imaging optics;

FIG. 17( a) shows how the (post uniform amplification) frequencyconversion can be performed before the imaging optics; and

FIG. 17( b) shows how the (post uniform amplification) frequencyconversion can be performed after the imaging optics.

DETAILED DESCRIPTION

In accordance with one or more embodiments of the present invention,systems and techniques for laser-based marking are described herein. Inthe following detailed description, various specific details are setforth in order to provide a thorough understanding of variousembodiments of the present invention. However, those skilled in the artwill understand that embodiments of the present invention may bepracticed without these specific details, that the present invention isnot limited to the depicted embodiments, and/or that exemplaryembodiments of the present invention may be practiced in a variety ofalternative embodiments. In other instances, methods, procedures, andcomponents, which would be well-understood by one skilled in the arthave not been described in detail to avoid unnecessary and burdensomeexplanation.

Furthermore, various operations may be described as multiple discretesteps performed in a manner that is helpful for understandingembodiments of the present invention. However, the order of descriptionshould not be construed as to imply that these operations need beperformed in the order they are presented, nor that they are even orderdependent. Moreover, repeated usage of the phrase “in one embodiment”does not necessarily refer to the same embodiment, although it may.Lastly, the terms “comprising”, “including”, “having”, and the like, asused in the present application, are intended to be synonymous unlessotherwise indicated.

A laser marking system embodying aspects of the present invention mayuse a semiconductor-based, digitally-controlled and programmablemicro-electromechanical system (MEMS) device configured to operate as aspatial light modulator (SLM) by way of an array (e.g., thousands) ofindividually-addressable, tiltable micro-mirror pixels. One example ofsuch a device is known in the art as a digital micromirror device (DMD),which is available from Texas Instruments Inc. Those skilled in the artwill appreciate that aspects of the present invention are not limited toreflection-based SLMs being that other types of spatial light modulators(e.g., transmission type, as may use a liquid crystal display (LCD), canalso be used. Further, while exemplary embodiments of the presentinvention describes SLM based laser image amplification system (LIAS) incontext of laser marking, the image amplification technique can be usedfor projecting images used in other applications such as, but notlimited to, optical lithography, laser direct writing, laser materialtransfer method, large screen home and cinema theatre etc.

As shown in FIG. 1, a laser light source 50, e.g., multimode lasersource configured to provide a beam having a flat-top profile may bearranged so that a laser beam from source 50 strikes an SLM 12 (e.g.,the above-described DMD) at normal incidence, for example. The Lasersource 50 may be a continuous wave (CW) source or a pulsed laser source.

As one skilled in the art would appreciate, a laser beam (eitherfundamental mode or multi-mode) under Gaussian laser beam propagation,may be in a collimated state at its minimum beam waist location and thusin one example embodiment the incident laser light source may becontrolled by an optical collimator 111, such that the beam has itsminimum beam waist at a receiving surface of SLM 12. Example embodimentsof optical collimator 111 may be a fiber gradient index (GRIN) lenscoupled to a fiber carrying the incident beam, or may be one or morestandard collimating lenses. It is noted that for a multi-mode laserwith a non-Gaussian irradiance distribution, the location of the minimumspot size is the same as that of the fundamental mode with just the sizeof the beam changing at the receiving surface of the SLM 12.

As will be appreciated by one skilled in the art, a laser beam has aspatial distribution of irradiance across a laser beam cross-section.For example, if the laser used for marking purposes has a singletransverse mode (e.g., fundamental or TEM₀₀ mode), then the laserirradiance distribution has a Gaussian shape, as shown in FIG. 4. Itwill be appreciated that non-Gaussian irradiance distributions can alsobe implemented for laser beams. For example, multi-mode lasers or laserswith higher-order modes can provide beams having a non-Gaussianirradiance distribution. The non-Gaussian irradiance distribution of amulti-mode laser beam may be qualitatively compared with the Gaussianirradiance distribution of a fundamental mode beam through a parameterreferred to in the art as the M² factor. For the fundamental mode laserbeams having a Gaussian distribution, the value of the M² factor isgenerally equal or close to unity while non-Gaussian beam distributionsgenerally have M² values higher than unity.

FIG. 5 illustrates an idealized non-Gaussian irradiance distributionknown in the art as a flat-top (or mesa) laser beam irradiancedistribution. By way of comparison, laser irradiance having a Gaussiandistribution, as illustrated in FIG. 4, has a maximum value at a centerof the beam and then the laser irradiance decreases radially outwardsfrom the center of the beam in accordance with the Gaussian mathematicalfunction. As further shown in FIG. 5, in an ideal flat-top laser beam,the irradiance distribution is absolutely uniform over the beamcross-section and thus is uniform over each segment of the image/barcodeintended to be placed on a target, thus ensuring a consistently uniformmarking quality.

In accordance with example embodiments of the present invention, theinventor of the present invention innovatively proposes a laser markingsystem embodying a laser beam having a substantially uniformnon-Gaussian irradiance distribution (i.e., a flat-top irradiancedistribution but for nominal tolerances or deviations, as will bereadily understood by one skilled in the art) and an optical amplifier20 configured to provide a substantially uniform amplification acrossthe cross-section of the beam reflecting off the SLM 12 (DMD) (i.e., anoptical amplifier configured to preserve the flat-top irradiancedistribution of the beam). An optical isolator 15 may be disposedbetween SLM 12 and optical amplifier 20 to optically block amplifiedback reflections, if any, which could affect SLM 20.

In one example embodiment, multimode laser source 50 may be configuredto provide an M² value so that the irradiance distribution of the laserbeams from such a source can be considered to be a substantially uniformirradiance distribution. By way of example, a beam would be consideredsubstantially flat-top, when the M² value of the beam has a value ofapproximately at least two or higher and when the laser irradiance levelhas reasonable ripple/uniformity, e.g., has a ripple variation within arange of about ±10%.

FIG. 6 illustrates a non-ideal flat-top irradiance distribution, whichdespite having some irradiance variation can provide a sufficientlyflat-top in the central zone of the beam (e.g., may have a ripplevariation within a range of about ±5%). In another example embodiment tobe described in greater detail below, the laser source may be afundamental mode laser source supplying a beam having a Gaussianirradiance distribution, which is then converted to a flat-topirradiance distribution.

As shown in FIG. 2, a laser source 52 may be a fundamental mode lasersource supplying a Gaussian beam distribution. Laser source 52 may bearranged so that a Gaussian beam from source 52 is optically coupled topass through a beam shaper configured to cause an effect on theirradiance distribution of the beam. In one example embodiment, the beamshaper may be a Gaussian-to-flat-top beam distribution converter 55 sothat an output beam from converter 55 is received by SLM 12 (e.g., theabove-described DMD) at normal incidence, for example. TheGaussian-to-flat-top beam converter 55 may be a commercially availabledevice, such as refractive beam shapers, available from NewportCorporation, CA and LIMO, Lissotschenko Mikrooptik Gmbh, Germany. Itwill be appreciated that converter 55 is not limited to any particulartechnical modality for performing the Gaussian-to-flat-top beam shaping,and may include one or more optical elements, such as diffractiveoptical elements, one or more lenses with aspheric surfaces, etc. Itwill be appreciated that in alternative embodiments a diffuser may beused to provide the beam irradiance shaping functionality, i.e.converting the Gaussian beam to a flat-top beam.

It is noted that using certain beam shapers effect the coherence of thelaser. A Gaussian-to-flat-top beam distribution converter 55 does notadversely effect the coherent of laser beam but other beam converterssuch as diffusers adversely affect the coherence of a laser beam. Thusthe beam shapers can be used to give coherent, partially coherent orincoherent beams. In all cases, the SLM reflection based images getamplified through uniform amplification. With a coherent beam, theoverall system is considered to be coherent imaging optical system andanalyzed accordingly. Whereas with incoherent beam, the overall systemis considered to be incoherent imaging optical systems and it isanalyzed accordingly. Although, exemplary embodiments of the inventionare described in detail for coherent imaging, it is understood thatexemplary embodiments of the invention cover incoherent imaging systemsas well.

It will be appreciated that Gaussian-to-flat-top beam distributionconverter 55 may be integrated as part of the laser source 52 or it maybe a separate optical component located downstream from laser source 52.It will be appreciated that any of the above-described examplearrangements would advantageously eliminate the prior art marking issuesdiscussed earlier since in each case the irradiance distribution of thelaser beam incident on the target would be a non-Gaussian irradiancedistribution, and more particularly, a flat-top irradiance distribution.The foregoing beam irradiance shaping functionality to obtain flat-toplaser beams (either through use of a beam converter, diffuser, or amulti-mode laser source) coupled with uniform optical amplification isexpected to provide substantial operational improvements compared toknown DMD-based laser marking systems, which lack such beam irradianceshaping functionality.

In operation each micromirror (in the array of individuallycontrollable, tiltable mirror-pixels in SLM 12) can have two tiltstates. For example, a first state when the micromirror is tilted by anangle +θ and a second state when the micromirror is tilted by an angleof −θ with respect to the normal to the device surface. When themicromirror is in a tilt state set at +θ, the micromirror will reflectthe laser light in a direction towards a target object 14. Conversely,when the micromirror is set in a tilt state of −θ, the micromirror willreflect the laser light away from the target object, for example,towards an optical absorber/block 16, as shown in FIGS. 1 and 2.

Thus, when an example high resolution binary image is generated by SLM12 in response to control signals from a controller 18, respective onesof the individually-controllable array of micromirrors in SLM 12 will beset to a respective +θ tilt state for each bit corresponding to a logicone in a given binary image. Similarly, respective ones of theindividually-controllable array of micromirrors in SLM 12 will be set toa respective −θ tilt state for each bit corresponding to a logic zero inthe given binary image.

It will be appreciated that as target object 14 may be marked with anintended 2-D matrix code or a high resolution image after beingamplified by optical amplifier 20, the beam incident on absorber/block16 will contain an optical pattern that is the logical complement of thedata code matrix or complementary code information. Thus, for markingapplications where complimentary 2-D matrix codes may be desirable, theabsorber/block 16 may be eliminated and both beams (e.g., the first beamgenerated in response to a +θ tilt state and the second beam generatedin response to a −θ tilt state) can be used for marking products 14 ₁and 14 ₂, as shown in FIG. 3. That is, one beam may be used for markingthe original data code matrix and the other beam may be used for markingthe logical complement of the original data code matrix. In this lattercase, as can be appreciated in FIG. 3, in lieu of absorber/block 16, theoptical components to use would be essentially a duplicate of thoseshown in the optical path for the beam resulting when the mirrors are inthe +θ tilt state.

As previously discussed, the magnitude of the incident laser beamirradiance, if not maintained within appropriate levels, can damage SLM12. The inventor of the present invention has recognized (through use ofoptical devices configured to provide a beam having a flat-topirradiance distribution in combination with an optical amplifierconfigured to provide a substantially uniform amplification across thecross-section of the beam reflecting off the SLM 12 (DMD)) an innovativesolution to the problem of avoiding damage to the micromirror device andmaintaining uniform beam irradiance.

It is noted that aspects of the solution proposed by the inventor ofexemplary embodiments of the present invention are not contingent onbeam expansion and beam contraction. It is further noted that aspects ofexemplary embodiments of the present invention ensure that the incidentlaser beam profile comprise a flat-top irradiance distribution and that,after reflection off the SLM 12, optical amplifier 20 is configured toprovide uniform amplification across the laser beam cross-section sothat the flat-top irradiance distribution of the beam is maintainedafter being amplified. Thus, the final irradiance of the beam can besignificantly larger than the damage threshold of the SLM 12 throughuniform amplification and the data carried within the cross-section ofthe beam remains essentially unchanged. Also, it is noted that exemplaryembodiments of the invention analyse the effects of diffraction on theimaging and amplification system performance and ensure that thoseeffects are dealt with necessary engineering solutions. Consequently,aspects of the present invention, in a straightforward manner overcomethe practical spatial, diffractive and irradiance limitations that arisein the context of prior art marking systems using SLMs.

Consistent with the practical constraints of SLM 12, an appropriatelevel of energy (e.g., less than a predefined threshold level needed forreliable DMD operation) is established for the laser beam incident onthe SLM 12 so that such a device is not damaged and functions with asubstantially high level of reliability. In one example embodiment, acommercially available DMD (SLM 12) is rated to safely accept 10 W/cm²of incident optical power in an example wavelength range fromapproximately 420 nm to approximately 700 nm. Furthermore, in theexample case of a pulsed laser operation, a reliable operating energylevel of approximately 0.1 J/cm² has been reported for such a device.Thus, in this example case, the energy level of the incident laser beamshould be kept below this threshold energy level. It will be appreciatedthat the foregoing levels should be construed in an example sense andnot in a limiting sense.

FIG. 7 illustrates a respective example of a 2-D data code matrix as maybe constructed by SLM 12. Each respective micromirror in the +θ tiltstate will contribute to form an output laser beam that comprises anoptical pattern containing the data code matrix. This beam is directedtowards the optical amplifier 20. An optical isolator 15, as illustratedin FIGS. 1 and 2, is placed between the SLM and the optical amplifier 20in order to block any amplified back reflections that might damage theSLM. For the example case of the 2-D data code matrix shown in FIG. 7,FIG. 8 shows an example resulting irradiance profile of the beamreflecting off the SLM 12. FIG. 8 shows that the uniform/flat-top beamincident on the SLM 12 is spatially encoded with the pattern/imageinformation and that each of the segments of the pattern/image hasuniform irradiance level.

In accordance with aspects of exemplary embodiments of the presentinvention, optical amplifier 20 is configured to uniformly boost thelaser beam intensity, i.e., the amplification factor is uniform acrossthe whole beam cross-section. The uniform amplification ensures that allsegments of the output patter/image have higher irradiance level whilemaintaining irradiance uniformity.

FIG. 9, FIG. 10 and FIG. 11 are used to comparatively conceptualizeaspects of exemplary embodiments of the present invention relative tooptical amplification techniques generally used in prior art lasermarking system. In this example, the pattern/image shown in FIG. 8represents an example optical input to be optically amplified.

FIG. 9 shows an example output pattern/image when the amplifier designprovides non-uniform amplification, e.g., defined by a Gaussian function(see FIG. 4), across the beam cross-section, as is generally performedin prior art laser marking system. The output pattern irradiance fromsuch an amplifier is believed to provide marks of unacceptable qualityand in case of 2-D bar code patterns unreadable codes.

FIG. 10 shows an example output pattern/image when the amplifier design,in accordance with aspects of the present invention, provides idealuniform amplification, e.g., an ideal flat-top response, as illustratedin FIG. 5, across the beam cross-section. FIG. 11 shows an exampleoutput pattern/image when the amplifier design provides non-ideal yetsufficiently uniform amplification, e.g., defined by a flat-top responsethat has a ripple variation within a range of about ±10% across the beamcross-section, as illustrated in FIG. 6. It will be appreciated that theexample response illustrated in FIG. 10 comprise a theoretical response,and the example output pattern irradiance shown in FIG. 11 represents arealistic uniform amplification within a degree of acceptable variation.Thus, the optical design of optical amplifier 20 is configured toprovide uniform amplification across the cross-section of the incidentbeam carrying the encoded pattern/image. That is, the amplified laserbeam generated by optical amplifier 20 contains the same information(such as the earlier mentioned example of a data code matrix) asgenerated by spatial light modulator 12 but with a higher and spatiallyuniform irradiance.

It is contemplated that a system embodying aspects of the presentinvention can be advantageously used to provide either ablative ornon-ablative marks on substances that require higher irradiances formarking. As will be readily understood by one skilled in the art, anon-ablative mark may be achieved through color change of the actualmarked object or a coating, under the influence of the incident laserirradiance.

As will be appreciated by those skilled in the art, optical amplifiersare generally composed of a gain medium, pumping source/mechanism and/oroptics (such as lenses, optics, gratings etc). Optical amplifiers can bebroadly categorized in different classes, such as one-pass amplifiers,multi-pass amplifiers or regenerative amplifiers. The variousinterdependent factors such as degree of required amplification, gainand saturation properties of the gain medium, characteristics of theinput beam etc. have to be configured via amplifier optical design toobtain the desired results. The optical design provides the couplingbetween the pumping source and the absorbing gain medium. It is alsoresponsible for the pump power distribution in the gain medium which inturn influences the uniformity and optical distortions of the outputbeam.

In one example embodiment, a one-pass optical amplifier might be used tofunction as the optical amplifier. In a one-pass amplifier, the inputbeam passes just once through the gain medium which is uniformly pumpedby a pumping source to enable stimulated emission, as illustrated inFIG. 12. The gain medium of the optical amplifier, under stimulatedemission, adds a substantial number of photons to the input beamentering the amplifier, thus increasing the energy content of the outputbeam. In this way, the spatially uniform irradiance input beam withinformation encoded across its cross section is uniformly amplified. Theoutput beam has higher irradiance without significant loss in the signalto noise ratio (contrast ratio) for the information encoded across itscross section, i.e., all pixel have approximately the same level ofirradiance.

In another example embodiment, a multi-pass optical amplifier might beused as the optical amplifier. Since any additional pass through thegain medium of the laser amplifier will provide an incrementalamplification to the input beam, a multi-pass amplifier 30 having amirror-array 32 may be used as the optical amplifier, as illustrated inFIG. 13. In this case, the gain/amplification medium is also uniformlypumped by the pumping source. The array of bulk-mirrors 32 may beselectively positioned relatively to an incident passing laser beam(e.g., via a tilt control arrangement represented by twin-headed arrow34) to reflect the passing laser beam a number of times through theamplifying medium. As seen in FIG. 13, the passing beam propagatesthrough a different optical path across the medium each time to makeeffective use of the available volume of the amplifying medium. Amulti-pass optical amplifier might generally provide more amplificationthan a single-pass amplifier but it might also provide lower signal tonoise ratio. It will be appreciated that various optical amplifierarchitectures may be used in the system design, as long as they satisfythe necessary condition of providing uniform amplification across theinput beam cross-section.

Provided below are various examples of optical amplifiers and amplifyingtechniques that one skilled in the art may use for proving asubstantially uniform amplification across the cross-section of thebeam, so that the amplified beam maintains a substantially uniformirradiance distribution over its beam cross-section. The examples givenbelow should be construed as illustrative of state of the art inconnection with optical amplifiers configured to provide substantiallyuniform amplification to a laser beam. That is, they should be construedin an example sense and not in a limiting sense. For example, an opticalamplifier may provide uniform amplification where the gain medium ispumped uniformly. The gain media may be essentially uniformly pumpedusing a multimode optical source which results in uniform amplificationas well as uniform temperature profile in the gain medium. Having auniform temperature profile may be an added benefit since it avoids orreduces thermal distortions. An exemplary amplifier may have a solidstate gain medium and an arrangement of diode bars configured to provideuniform gain/amplification.

It will be appreciated by those skilled in the art that some of theforegoing examples are discussed in the context of providing uniformpumping to a laser cavity; however, those skilled in the art willappreciate that such examples can be readily adapted to pump an opticalamplifier.

Those skilled in the art would know that the size of elements in SLM 12and other high resolution SLMs is typically small, e.g. in micrometers,leading to optical diffraction effects. For proper design of a systemusing SLMs the diffraction effects have to be taken into consideration.For example, the SLM 12 may be a 2-dimensional array of periodicallyspaced mirrors. When the array is illuminated with a laser, it behavesas a reflective diffraction grating. In essence, the diffractionproduces multiple copies of the image generated by the SLM 12. However,by titling the SLM 12 through a certain angle, as illustrated in FIG.15, a majority of the incident laser power can be coupled into oneparticular diffraction order and majority of the incident laser powerwould be coupled into one. The chosen order/image could then beoptically amplified.

In an embodiment of the system in FIG. 1, coherent light or laser isused with an SLM where the SLM modulates the amplitude of the laser beamacross a cross-section of the beam and where the modulation signal isthe image created by SLM 12. The overall system is therefore a coherentoptical system which necessitates coherent optical system analysis basedon propagation and diffraction of coherent light. As described above,tilting the SLM 12 through an angle allows coupling majority ofreflected light into one diffracted order. The image contained in thisdiffracted order beam goes under diffraction as it propagates (underHuygens-Fresnel principal). An optical-conditioning system 22 capturesthe propagating diffracted beam and forms a magnified/demagnifiedversion of the SLM 12 image in the final image plane as maybe defined bythe lens equation. As the diffracted order beam propagates, theinformation it carries across its cross section changes due todiffraction. Right at the surface of the SLM 12, the information is theimage generated by the SLM 12 but before it enters theoptical-conditioning system 22 the information it carries can berepresented by the Fresnel diffraction pattern. After the beam haspropagated through the lens and approached the final image plane, theinformation it carries changes from the Fresnel diffraction pattern to amagnified/demaginified version of the SLM 12 generated image. Theinformation can be uniformly amplified before or after theoptical-conditioning system 22. FIG. 16( a) illustrates how performinguniform amplification before the optical-conditioning system 22 willamplify the Fresnel diffraction pattern and FIG. 16( b) illustrates howperforming uniform amplification after the optical-conditioning system22 will amplify the image. The net result in both cases is the same,which is the uniform amplified scaled version of the image generated bySLM 12.

Returning to FIG. 1, the optical-conditioning system may be composed ofcollimating and aplanat focusing lenses. As one skilled in the art ofoptics would recognize, an aplanat lens is designed to be substantiallyfree of spherical and/or coma wave-front errors or aberrations. Thepresence of either of these aberrations could distort an opticaltransmitting wave-front and could cause the final image on the surfaceof the marked test object to become irregularly shaped or blurred. Thefinal amplified image contains the optical pattern having an examplehigh resolution image or a 2-D matrix code imparted by SLM 12 (e.g., interms of intense laser light for logic one bits and no light for logiczero bits).

In one example embodiment, the optical-conditioning system 22 may bearranged to project and focus the optical pattern on the surface of thetarget object 14 to be marked. In one example embodiment, theoptical-conditioning system 22 may do so by capturing the beam with itsminimum beam waist at the surface of the SLM 12. In one exampleembodiment, the regions on the surface of the target object that receiveintense laser light (e.g., corresponding to logic one bits) get marked(or ablated) whereas other regions (e.g., corresponding to logic zerobits) experience no change. Thus, in this manner, the 2-D data codematrix is marked on the surface of the target object. Depending on theneeds of any given marking application, optical-conditioning system 22can be adjusted to magnify or reduce the size of the optical patternthat contains the 2-D code matrix or any other high resolution image.

In operation, the orientation of the micromirrors of SLM 12 and hencethe optical pattern that contains the 2-D code information can berapidly changed through the controller 18. If a given 2-D code can berepresented as a code frame, the ability of the SLM 12 to re-orient itsmirrors orientation and project new code frames at a substantially highframe rate, allows a image amplification system embodying aspects of thepresent invention to mark products with sequentially-changing codematrices and thus advantageously allows serialized laser marking ofproducts at substantially high speeds. The high marking speed wouldallow numerous industries to incorporate the marking system in existingindustrial production lines without affecting associated productionprocesses.

It will be appreciated that during the serialized (e.g., sequential)laser marking process, the DMD is stationary since its spatial lightmodulating operation is implemented through changes in the orientationof its micromirrors. In one example embodiment, the products to bemarked may be placed on a conveyer belt or a similar mechanism formoving objects and such products will be marked under the action ofspatially uniformly amplified laser irradiance as they move.

In one example embodiment, SLM 12 can operate in various regions of thefrequency spectrum of light, such as ultraviolet (UV), visible andnear-infrared (IR) spectral ranges. Thus the wavelength of the incidentlaser beam can be suitably adjusted based on the type of material beingmarked. It will be appreciated that various characteristics of the laserirradiance induced mark may be similarly adjusted. By way of example andnot of limitation, other laser parameters that may be suitably adjustedmay include energy/power, spot size (for both continuous wave and pulsedlasers), pulse width, pulse repetition rate and number of pulses (forpulsed lasers).

It is mentioned earlier that, SLM 12 can operate in various regions ofthe frequency spectrum of light, such as ultraviolet (UV), visible andnear-infrared (IR) spectral ranges. However, at shorter opticalwavelengths, especially UV, the energy of the photons is much higher andtheir capability to damage the SLM 12 is also higher which is why thedamage thresholds are much lower for the SLM 12 at shorter wavelengths.This limits the performance characteristics and in some cases the veryviability of a lot of application systems that use image projection atshorter wavelengths. The laser image amplification technique can be usedto generate and uniformly amplify an image using a higher wavelength andthen using non-linear optics principles (frequency conversion) convertthe laser and the image to a shorter wavelength. For example, 1064 nmwavelength laser incident on the SLM 12 generates an image which thengets uniformly amplified. The amplified 1064 nm image can then beconverted to a 532 nm (visible green) wavelength image using frequencyconversion non-linear optics/medium (frequency doubling) or to 355 nm(UV) wavelength image (frequency tripling). In this way, the SLM 12 onlyhandles a longer wavelength with lower damaging strength but the finalimage is of a shorter much more powerful wavelength. This enablesapplications to use projected images with much higher irradiance levelsat shorter optical wavelength such as UV. The frequency conversion canbe performed using a frequency conversion device, such as, but notlimited to, a non-linear optical (NLO) medium 60, as illustrated inFIGS. 17( a) and 17(b), after amplification where the NLO medium 60 isphysically located before or after the optical-conditioning system 22.

Thus, FIG. 17( a) illustrates uniform amplification and subsequentfrequency conversion of the Fresnel diffraction pattern before theoptical-conditioning system 22 and FIG. 17( b) illustrates uniformamplification and subsequent frequency conversion of the image after theoptical-conditioning system 22. The net result in both cases is thesame, which is the uniform amplified scaled and frequency convertedversion of the image generated by SLM 12. Note, that in case of multiplepass uniform amplification, frequency conversion would be performedafter the last pass of the image carrying beam through the amplifier.

While the invention has been described with reference to variousexemplary embodiments, it will be understood by those skilled in the artthat various changes, omissions and/or additions may be made andequivalents may be substituted for elements thereof without departingfrom the spirit and scope of the invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the invention without departing from the scope thereof.Therefore, it is intended that the invention not be limited to theparticular embodiment disclosed as the best mode contemplated forcarrying out this invention, but that the invention will include allembodiments falling within the scope of the appended claims. Moreover,unless specifically stated, any use of the terms first, second, etc., donot denote any order or importance, but rather the terms first, second,etc., are used to distinguish one element from another.

1. A laser marking system comprising: a laser apparatus configured tosupply a laser beam having a non-Gaussian irradiance distribution over abeam cross-section, wherein the non-Gaussian irradiance distribution ofthe laser beam comprises a substantially uniform irradiance distributionover the beam cross-section; a spatial light modulator optically coupledto receive the laser beam, wherein the spatial light modulator iscontrolled to generate an output laser beam comprising an opticalpattern across the beam cross-section to mark a target object with thedata code matrix; and an optical amplifier coupled to the spatialmodulator to receive the laser beam output from the spatial lightmodulator and generate an amplified laser beam such that a final opticalpattern is an amplified version of the optical pattern as generated bythe spatial light modulator, the amplified laser beam from the opticalamplifier having a substantially uniform amplification across thecross-section of the beam, thereby the amplified beam maintaining thesubstantially uniform irradiance distribution over its beamcross-section.
 2. The laser marking system of claim 1, wherein the laserbeam with the substantially uniform irradiance distribution has a M²value that exceeds a predefined threshold value.
 3. The laser markingsystem of claim 2, wherein the laser beam with the substantially uniformirradiance distribution has a ripple variation within a predefinedrange.
 4. The laser marking system of claim 1, wherein the laserapparatus comprises a multimode laser source configured to supply thelaser beam having the substantially uniform irradiance distribution. 5.The laser marking system of claim 1, wherein the laser apparatuscomprises a fundamental mode laser source configured to supply a laserbeam having a Gaussian irradiance distribution, wherein the laserapparatus further comprises a Gaussian-to-non-Gaussian irradiancedistribution converter that receives the laser beam having the Gaussianirradiance distribution and is configured to supply the laser beamhaving the substantially uniform irradiance distribution over the beamcross-section.
 6. The laser marking system of claim 5, furthercomprising an optical collimator arranged to collimate the laser beamsupplied by the fundamental mode laser source.
 7. The laser markingsystem of claim 6, wherein the optical collimator comprises agradient-index (GRIN) lens optically coupled to an optical fiber thatcarries the laser beam supplied by the fundamental mode laser source tothe Gaussian-to-non-Gaussian irradiance distribution converter.
 8. Thelaser marking system of claim 6, wherein the optical collimatorcomprises at least one collimator lens.
 9. The laser marking system ofclaim 1, wherein the laser apparatus comprises a fundamental mode lasersource configured to supply a laser beam having a non-uniform irradiancedistribution (e.g. a Gaussian distribution), wherein the laser apparatusfurther comprises an optical diffuser that receives the laser beamhaving the non-uniform irradiance distribution and is configured tosupply the laser beam having the substantially uniform irradiancedistribution over the beam cross-section.
 10. The laser marking systemof claim 1, wherein the optical pattern comprises a data code matrixand/or a logo.
 11. The laser marking system of claim 1, furthercomprises an optical-conditioning system to capture a propagatingdiffracted beam of the optical pattern and to form amagnified/demagnified version of the optical pattern.
 12. The lasermarking system of claim 11, wherein the optical-conditioning systemcaptures the propagating diffracted beam between the spatial lightmodulator and the optical amplifier or between the optical amplifier anda target area.
 13. The laser marking system of claim 1, furthercomprises a frequency conversion device to convert the amplified laserbeam and the optical pattern to a shorter wavelength when a higherwavelength is initially used.
 14. The laser marking system of claim 11,further comprises a frequency conversion device, wherein the frequencyconversion device provides for subsequent frequency conversion performedbefore the propagating diffracted beam which comprises the opticalpattern passes through the optical-conditioning system wherein thesubsequent frequency conversion is perform on a Fresnel diffractionpattern of the optical pattern.
 15. The laser marking system of claim11, further comprises a frequency conversion device, wherein thefrequency conversion device provides for subsequent frequency conversionperformed after the propagating diffracted beam which comprises theoptical pattern passes through the optical-conditioning system whereinthe subsequent frequency conversion is performed on the actual opticalpattern.
 16. A laser marking system comprising: means for generating alaser beam having a non-Gaussian irradiance distribution over a beamcross-section, wherein the non-Gaussian irradiance distribution of thelaser beam comprises a substantially uniform irradiance distributionover the beam cross-section; a spatial light modulator optically coupledto receive the laser beam, wherein the spatial light modulator iscontrolled to generate an output laser beam comprising an opticalpattern containing a data code matrix, a logo, or both across the beamcross-section to mark a target object with the data code matrix; andoptical amplifying means for generating an amplified laser beam suchthat a final optical pattern is an amplified version of the opticalpattern generated by the spatial light modulator, the amplified laserbeam from the optical amplifying means having a substantially uniformamplification across the cross-section of the beam, thereby theamplified beam maintaining the substantially uniform irradiancedistribution over its beam cross-section.
 17. The laser marking systemof claim 16, wherein the laser beam with the substantially uniformirradiance distribution has a M² value that exceeds a predefinedthreshold value.
 18. The laser marking system of claim 16, wherein thelaser beam with the substantially uniform irradiance distribution has aripple variation within a predefined range.
 19. The laser marking systemof claim 16, wherein the means for generating the laser beam comprises amultimode laser source configured to supply the laser beam having thesubstantially uniform irradiance distribution.
 20. The laser markingsystem of claim 16, wherein the means for generating the laser beamcomprises a fundamental mode laser source configured to supply a laserbeam having a Gaussian irradiance distribution, the laser beam suppliedby the fundamental mode laser source optically coupled to aGaussian-to-non-Gaussian irradiance distribution converter configured tosupply the laser beam having the substantially uniform irradiancedistribution.
 21. The laser marking system of claim 16, furthercomprising an optical collimator arranged to collimate the laser beamsupplied by the fundamental mode laser source.
 22. The laser markingsystem of claim 21, wherein the optical collimator comprises agradient-index (GRIN) lens optically coupled to an optical fiber thatcarries the laser beam supplied by the fundamental mode laser source tothe Gaussian-to-non-Gaussian irradiance distribution converter.
 23. Thelaser marking system of claim 21, wherein the optical collimatorcomprises at least one collimator lens.
 24. The laser marking system ofclaim 16, wherein the means for generating the laser beam comprises afundamental mode laser source configured to supply a laser beam having aGaussian irradiance distribution, the laser beam supplied by thefundamental mode laser source optically coupled to an optical diffuserconfigured to supply the laser beam having the substantially uniformirradiance distribution over the beam cross-section.
 25. The lasermarking system of claim 16, wherein the spatial light modulator isselected from the group consisting of a reflection-based spatial lightmodulator and a transmission-based spatial light modulator.
 26. Thelaser marking system of claim 16, further comprises anoptical-conditioning system to capture a propagating diffracted beam ofthe optical pattern and to form a magnified/demagnified version of theoptical pattern.
 27. The laser marking system of claim 26, wherein theoptical-conditioning system captures the propagating diffracted beambetween the spatial light modulator and the optical amplifier means orbetween the optical amplifier means and a target area.
 28. The lasermarking system of claim 16, further comprises a frequency conversiondevice to convert the laser beam and the optical pattern to a shorterwavelength when a higher wavelength is initially used.
 29. The lasermarking system of claim 16, further comprises a frequency conversiondevice wherein the frequency conversion device provides for subsequentfrequency conversion performed before the propagating diffracted beamwhich comprises the optical pattern passes through theoptical-conditioning system and wherein the subsequent frequencyconversion is performed on a Fresnel diffraction pattern of the opticalpattern.
 30. The laser marking system of claim 16, further comprises afrequency conversion device, wherein the frequency conversion deviceprovides for subsequent frequency conversion performed after thepropagating diffracted beam which comprises the optical pattern passesthrough the optical-conditioning system wherein the subsequent frequencyconversion is performed on the actual optical pattern.